Born December 6 in Yorkshire, William Cavendish was a man of many interests and talents. He fought as a military leader for the royalists, served as adviser to Charles I and Charles II, wrote plays and poems, and founded a riding school. During the interregnum, William and his brother Charles spent several years in Paris, where they met Hobbes, Gassendi, and Descartes, among others. While in France, William married his second wife, Margaret (1623–73), a writer and philosopher who published works critical of mechanical philosophy in general and of Descartes (among others) in particular. Both Cavendish brothers corresponded with Descartes, and three letters from Descartes to the marquess have survived (AT IV 188–92, 325–30, and 568–77; CSMK 274–76, 302–4). In the second of these, Descartes remarks that the chief goal of his studies is the preservation of health, and he seconds Tiberius's claim that anyone over thirty is qualified to be his or her own physician, inasmuch as good health depends upon being attentive to one's experience of what harms or benefits one's body (AT IV 329–30, CSMK 275–76). In the third letter, Descartes repeats his claim from the Discourse on Method that animals are nothing more than unthinking, self-moving machines. He bases this on the grounds that because (according to him) language use is the surest sign of a thinking mind, and no animals use language, while even the slowest human beings do, animals must be without thoughts. It is interesting that in the Discourse (AT VI 59–60, CSM I 141) Descartes concluded that if we grant that animals think just as we do, then we have no more hope of an afterlife than does a fly or an ant (i.e., none); because we do have such a hope, it must be that they do not have minds. In his letter to Newcastle, on the other hand, he claims that if we attribute thought to animals, we must thereby attribute immortal souls to them (see soul, immortality of the). Given (Descartes says) that the idea that oysters and sponges might have immortal souls is clearly false, it must be that they do not have minds (AT IV 576, CSMK 304). Thus, Descartes arrives at the same conclusion via two similar, but distinct, lines of implication. William Cavendish died December 25, 1676, at Welbeck Abbey.

See also Animal; Body; Cavendish, Margaret; Discourse on Method; Language; Mind; Soul, Immortality of the; Thought